The polymerization of tetrafluoroethylene in aqueous medium can be carried out by two distinctly different procedures. In one procedure, little or no dispersing agent is employed and agitation is carried out vigorously in order to produce a precipitated resin, commonly referred to as "granular" resin. In the other procedure, sufficient dispersing agent is employed and agitation is mild in order to produce small colloidal size particles dispersed in the aqueous reaction medium. In this second procedure, called aqueous dispersion polymerization, precipitation (i.e., coagulation) of the resin particles is avoided. The two procedures produce distinctly different products. The granular product can be molded in various forms, whereas the resin produced by the aqueous dispersion method cannot be molded but is fabricated by dispersion coating or by converting to powder for paste extrusion with a lubricating medium. In contrast, the granular resin is incapable of being paste extruded or dispersion coated.
In aqueous dispersion polymerization it is desirable to obtain small size colloidal polytetrafluoroethylene particles to achieve good stability against coagulation from the dispersion, to facilitate penetration by the particles of woven interstices when the dispersion is coated on woven fabric, and to attain high extrusion pressures to enhance strength of paste extruded articles.
It is also desirable in aqueous dispersion polymerization to obtain high solids concentration, i.e., a high percent of polymer in the dispersion, because when the amount of polymer formed during the polymerization is increased, the production is increased.
Attempts have been made in the art to increase one or the other of these two desirable features. For example, British Pat. No. 1,246,834 sought to increase solids content by employing certain cispersing agents, while British Pat. No. 1,397,931 obtained fairly high solids content by employing a persulfate or H.sub.2 O.sub.2 initiator and by adding disuccinic acid peroxide at a certain point in the course of the polymerization. Punderson U.S. Pat. No. 3,391,099 teaches that controlled addition of dispersing agent aids in controlling particle size and solids content. However, as solids content increases, particle size also ordinarily increases, and research has continued toward finding a procedure for producing high solid content while producing small particles.